The invention concerns a process to determine the biological oxygen demand (BOD) of sewage, whereby a sewage sample is mixed with a biologically neutral dilution water at a predetermined degree of dilution and the oxygen concentration is measured in a biological bath which changes as a result of the biological reaction of the predetermined bio-mass with the diluted sewage sample in the biological bath.
The biological oxygen demand (BOD) represents an important parameter to indicate the degree of filth in sewage, because the filth loading of sewage predominantly depends on biologically degradable substances. The common idea in the known processes to determine this degree of filth is that a bio-mass, consisting of aerobic micro-organisms usually placed on a growth surface, is nourished in a biological bath with the sewage to be evaluated or a dilution thereof. The oxygen consumption occurring as a result of the biological reaction leads to a diminishment of the oxygen concentration in the biological bath which is measured and drawn to determine the biological oxygen demand and thus the degree of filth of the sewage to be analyzed.
In a known process of the type named at the beginning (EP-B-0 049 887) which, in contrast to the previously known, time-consuming processes, brought a basic improvement and acceleration of BOD determination, a sample stream of the sewage to be evaluated was diluted before its continuous entry into the biological bath by means of the constantly measured introduction of dilution water at a predetermined degree of dilution. The oxygen concentration was measured before entry into the biological bath and after its exit from the biological bath. The average measured value was kept constant at a predetermined value by changing the degree of dilution. This degree of dilution serves to determine the biological oxygen demand. The turbulent circulation of the bio-mass in the biological bath, which is required for a satisfactory reaction speed, occurs by means of the continuous pumping in a loop of the fluid contained in the biological bath.
The device required for the conduct of this process is relatively complicated, because a regulated amount of the sample stream and the dilution water is required. Measurement of the oxygen must occur at two locations. An additional pump is required to pump the fluid out of the biological bath.
It is therefore the task of this invention to so formulate a process of the type named at the outset, that it can the performed simply, quickly and with little requirement for equipment.